This invention relates to a demodulator for hybrid or quadrature amplitude modulation.
In hybrid modulation, such as described by R. W. Lucky in his book entitled "Principles of Data Communication," pp. 177-178, a cosine and a sine carrier signal are amplitude modulated by a first and a second signal derived from an input signal to be transmitted. It is already known that the hybrid modulation provides from twice to four times as much transmission capacity as compared to conventional frequency modulation, as discussed by D. D. McRae in IEEE Transactions on Communications, 1971 August Issue, pp. 431-445, under the title "Performance Evaluation of a New Modulation Technique." In addition, the hybrid modulation is believed to be effective for use in satellite communications. It is, however, difficult with conventional hybrid modulation techniques to control the phase of a local carrier signal produced on a receiving side.